Kiwiprop Feathering Propeller
The Kiwiprop feathering propeller is gradually gaining popularity in a competitive marketplace. It is set apart from its rivals by its construction—the blades are made of Zytel, a plastic composite made by Dupont, mounted on a 316 stainless-steel hub. This combination lessens the potential for galvanic corrosion, which was the main reason I decided to replace my bronze two-bladed folding propeller with the Kiwiprop. The zincs on our boat’s saildrive leg had been disappearing at an alarming rate, and I worried that the leg itself might start to corrode. I also hoped a three-bladed prop would improve the boat’s mediocre motoring performance.
Ostara displaces 12,000 pounds and her Yanmar 2GM20 engine should by rights have been a 3GM30; she is underpowered, and the folding prop could not drive her at much more than 4 knots into any kind of a head sea or wind. In flat water, with no wind, she would reach maybe 5.5 knots with the engine hammering away at close to its 3,600 maximum rpm. At a more sedate 2,900rpm (you should not run a diesel for long periods at more than 80 percent of peak rpm), she struggled to top 5 knots. The boat lives on a mooring, so I was not particularly concerned with reversing performance. Just as well, really, because it wasn’t good.
The Kiwiprop’s engineering intrigued me. I’ve always thought that folding propellers are works of metallurgical art, with complex gears, meticulous machining and gorgeous curves making them beautiful examples of the merging of form and function. The Kiwiprop is blunt and businesslike by comparison. Like other feathering propellers its blades are flat, not shaped, and they feather individually so (in theory) they are always aligned with the water flow. In reverse, the blades flop over until they hit metal stops. The composite blades pivot on titanium pins set into the 316 stainless steel hub assembly, and removing and replacing them is the work of a few minutes—just as well, because you occasionally do need to take them apart for lubrication. Pitch can be altered by tweaking a set screw on each blade. Installation, in the case of my Saildrive, took all of 15 minutes, and that was working slowly.
Kiwiprop inventor John Blundell recommended a 15.5in prop with 20 degrees of pitch for Ostara. This is the smallest-diameter Kiwiprop – the maximum size is 18in, and they can be ordered to fit shaft sizes from 7/8in to 1 1/4in, in left- or right-handed rotation. My folding propeller was a 15 x 10, and the comparative lack of pitch explains the poor top end and the less-than-stellar performance in reverse.
From the moment we motored away from the dock, it was obvious that we had a different propeller. Propwalk in astern had never been a significant factor because the saildrive leg was so close to the boat’s pivot point, but now it was negligible. The boat stopped much more quickly when put into reverse and sternway could be maintained at tickover speed. Going forward, the coarser pitch manifested itself in a second’s delay before the prop bit after the engine was put in gear, but then the boat picked up speed much more rapidly than before.
I’ve now finished my third season with this propeller and I’m reasonably pleased with the performance. We can cruise at a little over 6 knots under power, though high 5s are more usual. With so little horsepower on tap, it’s noticeable that a dirty bottom has a significant effect on speed under power, more so than under sail. More importantly, Ostara can now punch into 20-plus knots of wind and choppy seas at a good 5 knots, which was not possible before.
Until the 2009 season I could not fault the prop, but last year the engine died almost every time I engaged reverse gear. Unless the engine was very well warmed up, it faltered and stalled before enough power could be applied to really get the prop spinning. There is a section on the Kiwiprop website dedicated to this problem, which is not uncommon with low-powered engines like my 18-horsepower 2GM. The most likely scenario appears to be either that the internal torsion spring tension is too high, meaning that the blades start developing thrust in reverse before they are fully open and thus overload the engine, or that the blades are binding on their pins; since I forgot to grease them before launching, this is the more likely reason. Either way, a strip-down and greasing will hopefully sort the problem out.
Kiwiprop also suggests that the engine may be losing power because of a carbon buildup in the engine exhaust elbow, a possibility intriguing enough that I plan to remove and clean this fitting over the winter. It could probably do with a good scraping anyway.
One question on my mind was the toughness of the blades. One day I managed to get a spinnaker guy wrapped around the propeller. The guy snapped a 3/16in stainless steel lifeline with a 3,000 pound breaking strain as if it were string, but the prop was unmarked. The blades are designed to snap if they stroke a hard object, and if I were heading off on a long cruise I would take a spare blade with me. Blades can be replaced with the boat in the water.
My overriding concern, corrosion of the saildrive leg, is no longer a worry. When we hauled the boat after its first season with the Kiwiprop, the zinc was hardly touched. Check out the two photos to see the difference. Now I feel bad about throwing out scarcely used $30 zincs every fall.
For more on the Kiwiprop, click here.